These tacks are more or less mandatory. The rest is discretionary; tacking on shifts. And small ones at that. This discussion is about using wind oscillations which are temporary shifts in the wind. The wind is shifting left and right but its average direction remains constant. In this kind of condition it is faster to maximize your Vmg up the average wind direction rather than the shifted wind direction. This is the principle behind the Wally.

Tacking on a shift vs. not tacking

The advantage of tacking

Here we see 2 identical boats on port. Green Machine detects a small shift and tacks onto starboard. Dead Red doesn’t notice or believe and keeps going, bearing away to the new wind because his telltales tell him to.

Notice that Green’s Vmg up the average wind (see below) increases while Red’s decreases. The equivalent straight-line boatspeed advantage is considerable;

A 5º shift is equivalent to about 1 knot of boatspeed or 100 seconds/mile!

The bigger it is, and the longer it lasts, the more seconds you gain.

Needless to say, having wind instruments helps you find those small shifts. Accurate wind instruments give you the confidence to believe what they are telling you.

The disadvantage of tacking

The downside is that tacking carries with it a certain amount of risk and penalty.

Distance lost to weather (DLw). All tacks produce a loss, and blown tacks are always a possibility. The expected gain needs to pay back this loss before it becomes one.

The wind shift peters out before you make back your tack loss.

You certainly shouldn’t tack if you won’t make up for the DLw. So how do you decide this?

The first thing is to know what a tack will cost you. See Tack Scoring for guidance on this, and Tack Tracker for the needed tools.

The second thing is deciding how big the shift is and how long it is likely to last. The bigger and longer the shift, the safer it is to use it. See Wind Statistics for information on how to guess better.

In addition to having good instruments to tell you about the wind shifts, the two factors – DLw and shift metrics control whether you should tack or not.

Gaining from shifts without tacking

There is also a way to take advantage of wind shifts without tacking. It has no DLw penalty, although the gain is less. The Wally was invented in Fremantle where the wind shifts were seemingly regular, but too short to exploit. The basic idea of the Wally is that when the wind shifts back and forth at least once during a leg, you do better by maximizing Vmg up or down the average wind instead of the current wind. This change of perspective always increases your DLw relative to sailing Vmg on the current wind. And it also pumps your lateral separation in such a way that wind shifts always help you and hurt them.